Ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi2Te3 thermoelectric films

The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi2Te3 thermoelectric films are studied through transient differential transmission spectroscopy. An ultralow frequency coherent optical phonon at 0.16 THz emerges, especially in ultrathin films, and it is ascribed to interlayer breathing modes. It can divide the ultrathin films into two groups which have out-of-phase vibration along the normal of a film plane, causing a destructive interference between in-plane propagating thermal waves in the two groups of quintuple layers, and thus possibly reducing the thermal conductivity of the ultrathin films. The excitation power dependence of ultrafast dynamics reveals carrier-carrier scattering dominating thermalization, which provides a microscopic understanding of the reported high electrical conductivity and anomalously high power factor of ultrathin Bi2Te3 films at room temperature.